In processes using high pressure steam, such as conventional and nuclear power plants, the recirculated steam contains iron oxide and other corrosion products. Normally, the sodium concentration in the recirculated steam condensate is below the levels at which it adversely affects operations. However, the sodium concentration can increase to the point where it fouls turbine blades when there is leakage of cooling water from condenser tubes into the recirculated steam condensate, reducing turbine efficiency. Thus, it is usually necessary to install facilities to demineralize condensate during periods of high sodium concentration.
In many installations it has been common practice to pass all condensate through ion exchange beds when, for the most part, these beds merely operate as filters for removing undissolved matter, such as iron oxide and other undissolved matter (normally referred to as "crud"), from the system. Since it is prohibitively expensive to discard ion exchange resin after it has become fouled in a system in which all condensate is circulated through the resin bed, ion exchange regeneration facilities normally are installed. In addition to the large capital investments required, these regeneration facilities also use relatively large quantities of regenerants, such as sulfuric acid and sodium hydroxide. Moreover, conventional deionization systems, both regenerable and nonregenerable, usually are not capable of producing an effluent below 0.5 ppb sodium. In many installations the condensate purification equipment is by-passed during periods of low sodium/chlorides concentration to minimize the sodium concentration in the condensate and to reduce regenerant consumption. However, during this by-pass period filterable contaminants are not removed from the condensate.
In other prior art condensate purification installations, filter septa or elements precoated with ion exchange resin are utilized to remove crud and deionize the condensate. These installations may be somewhat complex involving precoating tanks and precoat pumping facilities. Care must be taken to produce a substantially uniform precoat over the filters to prevent channeling and poor fluid flow distribution through the filters. Moreover, if the precoat layer is too thick, excessive pressure drop results, necessitating the installation of larger, more expensive pumps. In addition, removal and replacement of fouled or clogged filter elements or septa can be burdensome and time consuming.
In still other systems, the condensate is passed through electrically charged stainless steel beads to set up a magnetic field in which metallic impurities are trapped by the beads. This system has several disadvantages including high initial cost, high operating cost and inability to deionize the fluid processed.
An object of the present invention is to install a condensate purification system having a low initial cost and a low operating cost.
Another object is to provide a relatively simple system which has the capability to filter condensate with or without deionization.
A further object is to provide a system for removing crud without increasing the sodium concentration in the condensate.
The above-noted objects are met in the system described and claimed herein. A conventional tank is charged with a crud filtering means, such as filtering beads, preferably being chemically inert beads such as stainless steel beads or plastic beads filled with a relatively dense material having a density differing from that of the ion exchange means to be utilized. During periods of low sodium or chlorides concentration, the condensate is passed through a tank containing only this filtering means, while during periods of high sodium or chloride concentration in the condensate, ion exchange means such as conventional ion exchange resin, can be selectively added to the tank to deionize the condensate. If the ion exchange density differs from that of the filtering means, the resin subsequently can be separated and removed by conventional methods for regeneration or disposal when it becomes fouled or exhausted, while the entrapped undissolved matter may be removed from the filtering means in place by conventional air scrubbing and backwashing methods.